WEDNESDAY, July 10 (HealthDay News) -- Eating a lot of oily fish or taking potent fish oil supplements may increase a man's risk of developing prostate cancer, new research suggests.

Moreover, marine sources of omega-3 fatty acids may also raise the risk for aggressive prostate cancer, according to the study by scientists at the Fred Hutchinson Cancer Research Center in Seattle.

"These anti-inflammatory omega-3s were associated with a 43 percent increased risk for prostate cancer overall, and a 71 percent increased risk in aggressive prostate cancer," said study lead author Theodore Brasky, a research assistant professor at Ohio State University Comprehensive Cancer Center in Columbus, who was at Hutchinson at the time of the study.

Aggressive prostate cancer is often fatal, he added.

Omega-3 fatty acids, found in fish such as salmon, trout and fresh tuna and in fish oil capsules, are widely reputed to have health benefits because of their anti-inflammatory properties.

But this new research, published online July 11 in the Journal of the National Cancer Institute, confirms damaging evidence reported in two prior studies.

Just why are these omega-3 fatty acids associated with prostate cancer? "That's the million dollar question," Brasky said.

Omega-3 fatty acids may have properties that aren't well understood and in high doses could cause oxidative stress, which can lead to DNA damage, possibly increasing the risk for prostate cancer, he speculated.

Oxidative stress plays a role in other cancers, Brasky said.

Although omega-3 fatty acids have been touted as beneficial to heart patients, results of a study published May 9 in the New England Journal of Medicine found they didn't live up to the claims.

In that study, Italian researchers reported that omega-3 fatty acid supplements did not reduce death from heart disease, heart attacks or strokes in people with risk factors for heart disease.

"These fatty acids have been promoted as a blanket anti-chronic disease fatty acid," Brasky noted. "But nutrition is more nuanced, as is disease occurrence. It's about time we stop talking about foods as good or bad and no gray area," he said.

Based on these and other findings, Brasky thinks men should probably moderate their intake of fatty fish and fish oil supplements.

"We are getting to the point where we don't see a lot of benefit for heart disease. Some of the enthusiasm for these fats has been premature," he added.

One expert cautions that these new findings don't show a cause-and-effect relationship between prostate cancer and omega-3 fatty acids.

"All of these studies on associations, which is what this is, are hypothesis-generating because they are looking back in time," said Dr. Anthony D'Amico, chief of radiation oncology at Brigham and Women's Hospital in Boston. "It's not a cause and effect."

The study would have to account for other risk factors for prostate cancer before it could be considered definitive, he said. These include family history, age and race, among others, D'Amico explained.

For the study, researchers used data from the Selenium and Vitamin E Cancer Prevention Trial, which found no benefit from either of these nutrients but an increase for prostate cancer for vitamin E.

The researchers compared blood levels of omega-3 fatty acids in more than 800 men later diagnosed with prostate cancer with blood samples from nearly 1,400 men who did not develop the disease.

The difference in blood levels of omega-3 fatty acids between the lowest- and highest-risk groups was about 2.5 percent (3.2 percent versus 5.7 percent) -- a gap larger than achieved by eating salmon twice a week, the researchers noted.

The investigators found that men eating the most fatty fish and taking the most fish oil supplements had an overall 43 percent increase in risk for all prostate cancer, compared with men eating the least fish or taking the fewest supplements. The risk for aggressive prostate cancer was 71 percent higher; for non-aggressive prostate cancer, the risk was 44 percent greater.